Temperature sensing using a device which exhibits an exponential relationship between temperature and voltage in response to an excitation current, such as, for example, a PN junction temperature sensor is well known. In CMOS circuits, such PN junctions commonly are implemented as diode-connected substrate bipolar transistors. Typically, such temperature sensing circuits comprise one or two PN junctions, for example, one or two bipolar transistors, and in general, the bipolar transistor or transistors are diode connected. Temperature sensing circuits which comprise two transistors can provide continuous temperature measurements, while those which comprise one transistor measure the temperature at discrete time intervals. The former type temperature sensing circuit are commonly referred to as continuous type sensing circuits, while the latter are referred to as switched current sensing circuits. All such temperature sensing circuits operate on the principle that the voltage developed across the base/emitter junction of a bipolar measuring transistor, or indeed, any other diode or PN junction, when the PN junction is excited by an excitation current, is complementary to absolute temperature (CTAT), and allowing for unquantifiable factors, such as process variations, is given by the following equation:                                           V            be                    ⁡                      (            T            )                          =                              kT            q                    ⁢          ln          ⁢                                    I              c                                      I              s                                                          (        1        )            where                Vbe(T) is the base/emitter voltage of the transistor at temperature T° Kelvin,        k is Boltzmann's constant,        T is absolute temperature in degrees Kelvin,        q is the electron charge,        Ic is the collector current of the transistor resulting from the excitation current, and        Is is the saturation current of the transistor resulting from the excitation current,        and the log function is the natural log.        
When the two transistors of a continuous temperature sensing circuit are excited by excitation currents, which cause the transistors to operate at respective different current densities, or if the transistor of a switched current temperature sensing circuit is sequentially excited by two excitation currents of different values, which cause the transistor to operate at two different current densities, the difference of the base/emitter voltages between the transistors or transistor operating at the different current densities is proportional to absolute temperature (PTAT) and is given by the equation:                               Δ          ⁢                                          ⁢                                    V              be                        ⁡                          (              T              )                                      =                              kT            q                    ⁢                      ln            ⁡                          (                                                                    I                    e1                                    ·                                      A                    2                                                                                        I                    e2                                    ·                                      A                    1                                                              )                                                          (        2        )            where                ΔVbe(T) is the difference of the base/emitter voltages of the transistor or transistors operating at the different current densities at temperature T° Kelvin,        k, T and q are the same as in equation (1),        Ie1 is the emitter current, in the case of a two transistor continuous temperature sensing circuit, of one of the transistors resulting from the excitation current applied to the transistor, and in the case of a switched current temperature sensing circuit resulting from the first excitation current applied to the transistor,        Ie2 is the emitter current, in the case of a two transistor continuous temperature sensing circuit, of the other transistor resulting from the excitation current applied to the transistor, and in the case of a switched current temperature sensing circuit, resulting from the second excitation current applied to the transistor,        A1 is the emitter area of the first transistor in the case of a two transistor continuous temperature sensing circuit, and in the case of a switched current temperature sensing circuit, the emitter area of the single transistor, and        A2 is the emitter area of the second transistor of a two transistor continuous temperature sensing circuit.        
In a switched current temperature sensing circuit the two terms A1 and A2 cancel out, thus leaving equation (2) as:                               Δ          ⁢                                          ⁢                                    V              be                        ⁡                          (              T              )                                      =                              kT            q                    ⁢          ln          ⁢                                    I              e1                                      I              e2                                                          (        3        )            
Equations (1) and (3) hold true for a switched current temperature sensing circuit, if one ignores the voltage offset due to the intrinsic base/emitter series resistance of the transistor on the base/emitter voltage. However, in order to take account of the intrinsic base/emitter series resistance of the transistor, equation (1) for a switched current temperature sensing circuit with a diode connected bipolar measuring transistor becomes                                           V            be                    ⁡                      (            T            )                          =                                            kT              q                        ⁢            ln            ⁢                                          I                c                                            I                s                                              +                                    I              e                        ⁡                          (                                                r                  te                                +                                  r                  tb                                            )                                                          (        4        )            where                Ie is the emitter current of the transistor resulting from the excitation current,        rte is the intrinsic emitter series resistance of the transistor, and        rtb is the intrinsic base series resistance of the transistor.        
However, equation (4) only holds true if the base/emitter voltage of the transistor is measured directly at the transistor. In many switched current temperature sensing circuits, it is not possible to measure the base/emitter voltage of the transistor directly at the transistor. This is so particularly where a temperature measuring transistor is located remotely of a measuring circuit of a switched current temperature sensing circuit, and the measuring transistor is coupled to the measuring circuit by a two wire connection. Such measuring circuits, typically comprise a current source circuit for providing the excitation currents for exciting the measuring transistor, and a signal processing circuit for processing the base/emitter voltages of the measuring transistor resulting from the excitation currents to produce an output voltage. The output voltage is indicative of the temperature of the measuring transistor. Thus, in such sensing circuits, the excitation currents are applied to the measuring transistor from the measuring circuit on the same two lines as the resulting base/emitter voltages generated in the measuring transistor are applied to the signal processing circuit. In other words, the base/emitter voltages of the transistor are applied to the signal processing circuit along the current path, along which the excitation currents are applied to the measuring transistor. In such switched current temperature sensing circuits, the voltages applied to the signal processing circuit, as well as including the actual base/emitter voltage of the measuring transistor resulting from the corresponding excitation current, also include a voltage component resulting from the intrinsic base/emitter series resistance of the transistor, and a voltage component resulting from the series resistance of the portion of the line forming the current path between a pair of sensing nodes in the current path in the measuring circuit, at which the base/emitter voltage is applied to the signal processing circuit.
In order to take account of such line series resistance, as well as the intrinsic base/emitter series resistance, equation (4) for a switched current temperature sensing circuit with a diode connected bipolar measuring transistor becomes:                                           V            be                    ⁡                      (            T            )                          =                                            kT              q                        ⁢            ln            ⁢                                          I                c                                            I                s                                              +                                    I              e                        ⁡                          [                                                (                                                            r                      te                                        +                                          r                      le                                                        )                                +                                  (                                                            r                      tb                                        +                                          r                      lb                                                        )                                            ]                                                          (        5        )            where                rle is the series resistance in the line forming the current path between the emitter of the measuring transistor and the corresponding sensing node in the current path on the emitter side of the measuring transistor, at which the base/emitter voltage is applied to the signal processing circuit,        rlb is the series resistance in the line forming the current path between the base of the measuring transistor and the corresponding sensing node in the current path on the base side of the measuring transistor, at which the base/emitter voltage is applied to the signal processing circuit,        rte is the intrinsic emitter series resistance of the measuring transistor, and        rtb is the intrinsic base series resistance of the measuring transistor.Equation (5) can be rewritten as follows:                                           V            be                    ⁡                      (            T            )                          =                                            kT              q                        ⁢            ln            ⁢                                          I                c                                            I                s                                              +                                    I              e                        ⁡                          (                                                R                  e                                +                                  R                  b                                            )                                                          (        6        )            where        Re=(rte+rle), namely, the sum of the intrinsic emitter series resistance of the measuring transistor, and the line series resistance in the emitter leg of the current path between the measuring transistor and the corresponding sensing node, and        Rb=(rtb+rtb), namely, the sum of the intrinsic base series resistance of the measuring transistor, and the line series resistor in the base leg of the current path between the measuring transistor and the corresponding sensing node.        
Hereafter, in the specification the term “current path series resistance” is intended to refer to the sum of the series resistance (Re+Rb), namely, the sum of the intrinsic base/emitter series resistance of the measuring transistor and the line series resistance in the current path between the measuring transistor and the sensing nodes.
However, in a switched current temperature sensing circuit comprising a bipolar measuring transistor having its collector connected to ground, account must be taken of the current gain of the measuring transistor. Accordingly, equation (6) for a bipolar measuring transistor with its collector connected to ground becomes:                                           V            be                    ⁡                      (            T            )                          =                                            kT              q                        ⁢            ln            ⁢                                          I                c                                            I                s                                              +                      Ie            ⁡                          (                                                R                  e                                +                                                      R                    b                                    β                                            )                                                          (                  6          ⁢          a                )            where                β is the current gain of the measuring transistor.        
In a switched current temperature sensing circuit which comprise a single measuring transistor, if a is the ratio between the two excitation currents which are alternately forced into the measuring transistor, the following is the relationship between the currents in the measuring transistor:   a  =                    I        c2                    I        c1              =                            I          e2                          I          e1                    =                        I          b2                          I          b1                    where                Ic1 and Ic2 are the two collector currents in the measuring transistor,        Ie1 and Ie2 are the two emitter currents in the measuring transistor, and        Ib1 and Ib2 are the two base currents in the measuring transistor,        all of which result from the two excitation currents.        
Accordingly, if N is the number of times the first excitation current is forced into the measuring transistor, and M is the number of times the second excitation current is forced into the measuring transistor, then the following equation can be derived from equation (6) where the measuring transistor is diode connected:                                                         MV              be2                        ⁡                          (              T              )                                -                                    NV              be1                        ⁡                          (              T              )                                      =                                                            k                ·                T                            q                        ⁡                          [                                                M                  ⁢                                                                          ⁢                                      ln                    ⁡                                          (                                                                        I                          c2                                                                          I                          s                                                                    )                                                                      -                                  N                  ⁢                                                                          ⁢                                      ln                    ⁡                                          (                                                                        I                          c1                                                                          I                          s                                                                    )                                                                                  ]                                +                                    (                                                MI                  e2                                -                                  NI                  e1                                            )                        ·                          (                                                R                  e                                +                                  R                  b                                            )                                                          (        7        )            Since Ie2=aIe1, equation (7) can be rewritten as:                                                         MV              be2                        ⁡                          (              T              )                                -                                    NV              be1                        ⁡                          (              T              )                                      =                                                            k                ·                T                            q                        ·                          ln              ⁡                              [                                                                            I                      c2                      M                                                              I                      s                      M                                                        ·                                                            I                      s                      N                                                              I                      c1                      N                                                                      ]                                              +                                                    I                e1                            ⁡                              (                                  Ma                  -                  N                                )                                      ·                          (                                                R                  e                                +                                  R                  b                                            )                                                          (        8        )            
Equations (7) and (8) for a switched current measuring circuit comprising a bipolar measuring transistor, having its collector connected to ground becomes:                                                         MV              be2                        ⁡                          (              T              )                                -                                    NV              be1                        ⁡                          (              T              )                                      =                                                            k                ·                T                            q                        ⁡                          [                                                M                  ⁢                                                                          ⁢                                      ln                    ⁡                                          (                                                                        I                          c2                                                                          I                          s                                                                    )                                                                      -                                  N                  ⁢                                                                          ⁢                                      ln                    ⁡                                          (                                                                        I                          c1                                                                          I                          s                                                                    )                                                                                  ]                                +                                    (                                                MI                  e2                                -                                  NI                  e1                                            )                        ·                          (                                                R                  e                                +                                                      R                    b                                    β                                            )                                                          (                  7          ⁢          a                )                                                                    MV              be2                        ⁡                          (              T              )                                -                                    NV              be1                        ⁡                          (              T              )                                      =                                                            k                ·                T                            q                        ·                          ln              ⁡                              [                                                                            I                      c2                      M                                                              I                      s                      M                                                        ·                                                            I                      s                      N                                                              I                      c1                      N                                                                      ]                                              +                                                    I                e1                            ⁡                              (                                  Ma                  -                  N                                )                                      ·                          (                              Re                +                                                      R                    b                                    β                                            )                                                          (                  8          ⁢          a                )            
In order to cancel the effect of the current path series resistance M a must equal N. However, to remove the dependence on Is, M must be equal to N. If M is equal to N, and if the effect of the current path series resistance is to be cancelled, then a must be equal to one. This could only happen if the two excitation currents were of equal value, and thus the result would be zero volts for all temperatures, since the log of one equals zero. Accordingly, from equation (8) it can be seen that in a switched current temperature sensing circuit using two excitation currents, one can either remove the dependence on the saturation current Is, or the voltage offset due to the effect of the current path series resistance, but not both at the same time.
A switched current temperature sensing circuit suitable for determining temperature of a measuring transistor, in which two excitation currents of different values are applied to a measuring transistor is disclosed in U.S. Pat. No. 6,097,239 of Miranda, et al. The temperature sensing circuit of Miranda provides an output voltage, which is derived from an accumulation of a plurality of voltage differences ΔVbe of the base/emitter voltage of the measuring transistor, resulting from excitations of the measuring transistor with the two excitation currents a number of times, and is thus indicative of the temperature of the measuring transistor. However, while the temperature sensing circuit of Miranda operates to remove the dependence on saturation current in the measuring transistor, the temperature sensing circuit of Miranda, since it operates with two excitation currents only, fails to cancel the effect of current path series resistance in the output voltage.
In order to remove the dependence on saturation current in a measuring transistor, and also to cancel the effect of current path series resistance in the output voltage of a switched current temperature sensing circuit, it has been suggested in U.S. Pat. No. 5,195,827 of Audy and Gilbert to force three excitation currents of different values sequentially through a measuring transistor.
However, low voltage switched current temperature sensing circuits are, in general, unsuitable for use with more than two excitation currents of different values. Typically, low voltage switched current temperature sensing circuits are implemented in CMOS, such as the temperature sensing circuits described in U.S. Pat. No. 6,097,239 of Miranda. The signal processing circuit which processes the base/emitter voltages of the measuring transistor resulting from the excitation currents for determining the voltage difference in the base/emitter voltage resulting from excitation with two different excitation currents, in general, comprise a switched capacitor integrating circuit which includes an operational amplifier (op-amp). The op-amp outputs a voltage which corresponds to the difference in the base/emitter voltages resulting from two or more excitations of the measuring transistor with the two excitation currents, and this voltage is indicative of the temperature of the measuring transistor. However, the voltage headroom in such op-amps, in general, is limited to a maximum of 5 volts. Since in general, the measuring transistor is biased at a voltage in the range of 0.2 volts to 0.7 volts, the remaining voltage headroom in the operational amplifier, in practice, is limited to approximately 4 volts.
The relationship between the base/emitter voltage of the measuring transistor and the excitation currents is a logarithmic relationship, and accordingly, a large ratio between the excitation currents is required in order to obtain a reasonable voltage difference between two base/emitter voltages resulting from excitations of the measuring transistor with the respective excitation currents. The ratio of the excitation currents should be such as to produce a voltage difference between the base/emitter voltages with an adequate signal to noise ratio in order to overcome inherent noise in the signal processing circuit. In such switched current temperature sensing circuits where the excitation currents are applied to the measuring transistor on the same lines as the base/emitter voltages are applied to the signal processing circuit, if the line series resistance is high, the base/emitter voltage applied to the signal processing circuit will include a significant voltage component resulting from the line series resistance, as well as a voltage component resulting from the intrinsic base/emitter series resistance of the measuring transistor, as has been discussed above. These voltage components are directly proportional to the entire series resistance in the current path through the base and emitter of the measuring transistor and the lines between the measuring transistor and the sensing nodes, across which the base/emitter voltage is applied to the signal processing circuit. The voltage components resulting from the entire current path series resistance is also directly proportional to the excitation currents. Thus, the higher the excitation current, and the higher the current path series resistance, the greater will be the ratio of the current path series resistance voltage components to the actual base/emitter voltage resulting from excitation of the measuring transistor, since the relationship between the base/emitter voltage and the excitation current is a logarithmic relationship. Since the voltage headroom of the operational amplifier in such temperature sensing circuits is relatively low, it is possible for the voltages in the op-amp during processing of the base/emitter voltages to exceed the voltage headroom. In such cases, the resulting output voltage of the op-amp will not be indicative of the temperature of the measuring transistor.
While the ratio of the excitation currents in such temperature sensing circuits can be selected, where two excitation currents are applied to the measuring transistor, in order to avoid exceeding the voltage headroom within the op-amp, in general, if such switched current temperature sensing circuits were to be used with three excitation currents of different values, the voltage headroom of the op-amp would be exceeded. The three excitation currents, namely, a high value current, a low value current, and an intermediate value current of intermediate value between the high and the low value currents, would have to be of such values as to provide a sufficient ratio between the intermediate value current and the high value current, on the one hand, and the intermediate value current and the low value current on the other hand, in order to provide corresponding base/emitter voltages with sufficient voltage difference between the respective base/emitter voltages. Due to the range between the high value excitation current and the low value excitation current, in general, the voltage headroom within the op-amp would be exceeded. This would be particularly so in the case of a switched current temperature sensing circuit in which the measuring transistor is located remotely of the measuring circuit, and the measuring transistor is coupled to the measuring circuit by a two wire connection, whereby the three excitation currents are applied to the measuring transistor along the same lines as the resulting base/emitter voltages are applied to the signal processing circuit of the measuring circuit.
There is therefore a need for a measuring circuit and a method for determining temperature from a PN junction temperature sensor which overcomes these problems and which minimises the effect of current path series resistance on the measured temperature value. Indeed, there is a need for a measuring circuit and a method for determining temperature of a device which exhibits an exponential relationship between temperature and voltage in response to an excitation current, in which the effect of current path series resistance is minimised, and there is also a need for a switched current temperature sensing circuit which similarly overcomes these problems, and in which the effect of current path series resistance is minimised.
The present invention is directed towards providing such a measuring circuit, a method and a switched current temperature sensor.